It appeared to be the latest chapter in last month's still unsettled presidential election.
The spokesman, Nqobizitha Mlilo, told The New York Times that about 200 to 250 heavily armed police officers raided the offices of the party, the Movement for Democratic Change, the apparently winning challenger.
It took place in a downtown building where opposition supporters were said to have taken refuge, fearing widespread political violence.
The diplomat, Jendayi Frazer, the assistant secretary of state for African affairs, said it appeared Mugabe sought to steal the election.
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